This invention relates to laminates and more particularly to laminates comprising polyamides and copolymers of styrene with acrylonitrile and to methods for their preparation.
Styrene-acrylonitrile (SAN) copolymer resins, including rubber-modified SAN resins and acrylonitrile-styrene-butadiene (ABS) graft resins, are widely employed in the form of extruded sheet material for manufacturing thermo-formed appliance housings, automotive panels, camper tops, boat hulls and the like. For many applications a co-extruded structural foam sheet having a core of foamed material and a smooth outer skin, usually formed from rubber-modified SAN copolymer resin, is employed. Whether in solid or foam-core sheet, SAN resins are widely accepted for a variety of applications where ease of thermo-forming, rigidity, toughness, good impact strength and good surface appearance and gloss are important considerations.
Although SAN resins have a highly desirable surface appearance and excellent resistance to staining, the surfaces withstand abrasion rather poorly, are subject to stress-crazing and chemical attack by hydrocarbon fuels, and are subject to severe deterioration on exposure to UV radiation. In end-uses where weatherability is an important factor, rubber-modified SAN resins must be protected either by means of effective stabilizers or by coating the exposed surfaces with acrylic sheet or other protective material. Alternative methods for modifying the surfaces with protective materials, though most frequently carried out for decorative purposes, include plating and painting. Both methods require that the surface of the SAN copolymer resin be treated in some manner to effect adhesion of the plating or paint coating. Further, the application of paint frequently results in deterioration of the mechanical properties, particularly the impact properties, of rubber-modified SAN sheet material.
The weatherability and environmental resistance of SAN copolymer resins may also be modified by forming blends with other resins to achieve a compromise balance of properties together with improved wear and resistance to chemical attack. For example, blends of ABS with nylon or with PVC are known to have high impact properties and good environmental resistance. These improvements may be gained at the expense of other highly desirable properties of SAN resins such as ease of thermoforming, and may result in substantial changes in such properties as flexural modulus and tensile strength.
Composites or laminates comprising two or more resin layers provide structures with altered surface characteristics without a significant change in the inherent mechanical properties of the individual components. As described herein above, a laminate comprising SAN copolymer resin base sheet and a thin acrylic cap sheet effectively resists exposure to UV radiation and weathering. The mechanical properties of the laminate are substantially those of the SAN base sheet, while the thin acrylic sheet provides surface protection and weatherability equivalent to that of acrylic sheet alone.
In principle, SAN base sheet could be provided with a tough, abrasion resistant surface by laminating the SAN sheet with a surface layer of a polyamide. Similarly, where excellent stain resistance and surface gloss and appearance are required, a polyamide base sheet could in principle be provided with a surface layer of SAN. In practice, laminates and composities of conventional polyamides and conventional SAN copolymer resins cannot be made due to the lack of adhesion between such dissimilar resins. Laminates formed of conventional SAN copolymers and polyamides literally fall apart, and hence are useless for nearly all applications.
Tightly-adhered composites of SAN copolymers and polyamides would provide structures having highly useful mechanical properties and desirable surface characteristics which would find extended utility over either resin alone.